The present invention relates to a thermal transfer sheet having a hot-melt ink layer that can be used a plurality of times with a printer for mainly printing character information.
A thermal transfer sheet comprising a substrate film and a hot-melt ink layer provided on one surface of the substrate film has hitherto been used as a thermal transfer recording medium for thermal printing, facsimile, etc. In the conventional thermal transfer sheet, paper having a thickness of about 10 to 20 .mu.m, such as capacitor paper or paraffin paper, or a plastic film having a thickness of about 3 to 20 .mu.m, such as a polyester film or a cellophane film, is used as a substrate film, and a hot-melt ink comprising a mixture of a wax with a colorant, such as a pigment or a dye, is coated on the substrate film to provide a hot-melt ink layer. The thermal transfer sheet is heated and pressed with a thermal head at a predetermined portion from the back surface of the substrate film to transfer the hot-melt ink layer at the predetermined portion corresponding to a printing portion to printing paper, thereby effecting printing.
In the above-described thermal transfer sheet, however, the hot-melt ink layer at its portion heated and pressed by the thermal head is entirely transferred to the printing paper by using the thermal transfer sheet only once, so that the number of times of printing with satisfactory results is only one in an identical portion, which leads to problems of low profitability due to large consumption of the thermal transfer sheet and high running cost.
For this reason, various thermal transfer sheets have been developed which could be used a plurality of times. Examples thereof include a thermal transfer sheet in which a transfer regulating layer comprising a thermoplastic resin is formed on the hot-melt ink layer to prevent the ink layer from being entirely transferred in the first printing; a thermal transfer sheet disclosed in Japanese Patent Laid-Open No. 165291/1985 in which a resin layer composed mainly of a polycaprolactone polymer is formed between the substrate film and the hot-melt ink layer; a thermal transfer sheet disclosed in Japanese Patent Laid-Open No. 11364/1988 in which the hot-melt ink layer at its portion heated and pressed with a thermal head through the substrate film gives rise to cohesive failure and is transferred to printing paper; a thermal transfer sheet disclosed in Japanese Patent Laid-Open No. 151483/1988 which comprises a first ink layer capable of being brought to a low-viscosity liquid upon heating and a second ink layer which is stickable to the first ink layer but cannot be brought to a low-viscosity liquid; and a thermal transfer sheet disclosed in Japanese Patent Laid-Open No. 16685/1989 which comprises a substrate film and, provided on the substrate film in the following order, a porous ink layer and an ink layer having a supercooling property.
However, all the above-described thermal transfer sheets have a problem that although the print density in the first printing is high, the print density in the second or later printing is rapidly lowered.
On the other hand, a two-color type thermal transfer material having a first ink layer and a second ink layer having a supercooling property is disclosed in Japanese Patent Laid-Open No. 152790/1987 and Japanese patent Laid-Open No. 249789/1987 although it does not aim to be used a plurality of times.
However, in the thermal transfer sheet provided with a second ink layer having a supercooling property simply laminated onto a first ink layer, the second ink layer is entirely transferred in the first printing, and the first ink layer is entirely transferred in the second printing, so that printing can be effected only twice at best.
Further, Japanese Patent Laid-Open No. 105514/1983 discloses a thermal transfer sheet having a hot-melt ink layer composed mainly of polycaprolactone. The melt Viscosity of the supercooling polycaprolactone as the main component is as high as 8000 to 15000 mPas, so that it is difficult to transfer the ink during printing, which gives rise to a problem that no good printing sensitivity can be obtained.
Under these circumstances, the present invention has been made, and an object of the present invention is to provide a thermal transfer sheet that can exhibit a high printing sensitivity and provide a homogeneous image even when it is used a plurality of times, and particularly to solve the problem of the conventional thermal transfer sheet for repeated use that the printing sensitivity of a print pattern, which is printed with a low printing energy, such as character information, is inferior to that of a printing ribbon for single printing.